Write-once read-many (WORM) optical media have stamped indications of manufacturer selected laser power level for recording or writing data on to the respective media. It is a current common practice to read and use such stamped indications for reading and writing data from and to a WORM disk. As such, upon loading a WORM disk into a WORM device, the laser is not initially calibrated in an in-focus condition at the recording layer of the WORM disk. The laser is initially calibrated in an out-of-focus condition to establish laser power to digital-to-analog convertor (DAC) relationship. This calibration enables the DAC to drive the laser to emit a beam power indicated by the known manufacturer's stamped indicated power level on the disk. Such calibration converts units used in the drive and factored by an estimated laser efficiency shift from an out-of-focus condition to an in-focus condition and a power to DAC relationship, such as set forth in the Call et al patent, supra. Bringing the laser beam into focus introduces an optical feedback via an external cavity effected by laser emitted light reflected by the optical medium. Such reflected laser light is coupled back into the laser as permitted by current optical heads used in magneto-optical recorders. Even though such optical feedback changes the lasing characteristics of laser operation, out-of-focus laser calibration avoids using write once data space for laser calibration thereby preserving write once data storage space to record data. For best operation it is desired to obtain better quality operation by in-focus calibrating a laser for write operations to a write once disk. Such calibration must be done in a manner to minimize usage of the limited write-once data storage areas on each given WORM optical disk.
Because of variability between optical media, media aging and laser control circuit aging, media and optical path contamination, actual emitted laser beam power for optimum recording may be different from laser drive current settings based upon such media-indicated laser beam power level. The above-mentioned aging may vary the operation of laser control circuits and media response resulting in either an undesired laser over-power level or laser under-power level situation. Such over-power laser beam levels can over ablate a track so as to obliterate adjacent recorded data, destroy a groove that interferes with tracking following and seeking, and the like. Such under-power levels may result in defective recording. Both error conditions may cause data losses. Therefore, it is desired to calibrate laser power level in a write-once media recorder to avoid such under or over power laser beam levels.
To accommodate the above-described variability between optical disk, The above-mentioned procedure of using a power level indicated by information stamped onto the disk can be approximated by calibrating a laser using an out-of-focus laser beam. Such out-of-focus calibration does not ablate the recording layer of a WORM disk. However, such out-of-focus calibration does not accurately reflect in-focus lasing, As a substitute for desired in-focus calibration, in-focus laser drive current values are calculated based upon out-of-focus (at the recording layer) measurements and an estimated efficiency shift of laser operation between such out-of-focus and in-focus changes in the laser beam.
Such out-of-focus calibration may result in a laser beam power level that is different than a desired laser beam level. That is, the laser beam power level for a given laser drive current changes as the beam is focused. This phenomena is caused by a shift in the differential efficiency of a semiconductor laser used in optical recording as the laser beam spot on an optical disk becomes focussed. Such shifts occurs because of light reflected from the disk into the laser cavity creating a cavity external to the laser. This shift in laser differential efficiency not only varies from laser to laser but also is affected by the efficiency of the optical feedback path. The optical path variability is caused by media variations and by contamination of the optical path (objective lens). Therefore, it is desired to calibrate and control laser beam power level in a write-once recorder using an in-focus beam and in a manner that data-storage space is not used.
In write-once media systems, it is not cost effective to use the media data-storing areas for calibrating a laser, such as is reasonable in rewriteable optical disks (usually magneto-optical). Since it is still desired to calibrate a laser beam power level in an in-focus condition, such in-focus calibration should be accomplished without unduly using data fields of disk sectors that would reduce the data-storage capacity of the disk.
In many write-once optical disks, a two-byte automatic laser power correction (ALPC) field is provided in each write-once disk sector. This ALPC field enables correcting laser power for writing data at a correct or desired emitted laser power level. The ALPC field also enables the laser to be operated at write level outside of any data area. Such writing of a laser test signal in the ALPC field is monitored for ensuring that the laser is emitting a proper level laser beam to the disk. Such monitoring is made with a photo detector receiving either the so-called wasted light from a beam splitter or using light from an auxiliary port of the laser. This ALPC write testing merely turned the laser continuously on at write level for a period of time less than time required to scan two bytes on the disk. Such an extended-time continuous write signal can have excessive energy resulting in so-called over-ablation (causes servo track following and seeking problems), i.e. the area ablated (physical size of the recorded laser test signal) exceeds the track width and may exceed the length of the ALPC field. Remember that such laser power level verification is measured at the output of the laser and does not measure ablation on an optical disk.
A reason for this undesired over ablation is that the duty cycle is different from a usual write pulse duty cycle. That is, a usual write pulse has a duty factor of about 10% that ablates about one-half of the track width. In contrast, the 100% duty cycle used in the laser ALPC write qualification is extreme and can ablate radially outside of the track being written to. Such over ablation not only extends radially but also circumferentially (at the trailing edge of the DC write pulse). While the duration of a recorded laser test signal need not fill the ALPC 2-byte field, many recorders do record such a laser test signal. In the latter instance, excessive laser power level results in a recorded laser test signal that crowds or extends to an ensuing write area, such as a sync area that precedes recorded user data. Recorded laser test signals having a shorter length may still radially over ablate in the ALPC area. Therefore, such extended continuous laser emissions may have undesired heating of the laser and its immediate environs. It is desired to avoid such over ablation.
The so-called correct write power level is also dependent on the duration or width of a laser write pulse. Writing in write-once media often assumes that the recorded write power level on each disk is correct. Because of circuit variations, signal delay tolerances of .+-.5%, signal propagation asymmetries in various circuits, and the like, unintended variation of actual write pulse widths (durations) may occur. Such pulse width variation has a non-linear effect on data recording in a WORM disk. It is desired to avoid such unintended variations of write-once recording.
Magneto-optical recorders also calibrate their respective lasers while in-focus for writing data. Such calibration requires many sectors for each calibration, as pointed out in the prior art below. It is important in WORM media to minimize the amount of data-storage space used for calibration of lasers. Therefore, the techniques used for magneto-optical recorders do not necessarily apply to calibrating lasers in WORM media drives or devices.